Almost forgot the cons, lol. About all I can think of is having a smaller antenna and therefor shorter range if your laptop does not have a connection avalaible for a built in antenna. My T42 has a large built in antenna so it performs much better than a pcmcia card would. If it didn't have that antenna I'm sure a pcmcia card would have longer range.
<A HREF="http://www.folken.net/myrig.htm" target="_new">My precious...</A>